1. Field of the Invention
The present invention relates generally to a thrust reverser, and more particularly, pertains to a blockerless thrust reverser for an aircraft having a podded nacelle housing a turbo-fan engine.
2. Description of the Prior Art
Existing thrust reversers require a mechanical blocker device to divert the normally aft-flowing fan stream and turn it outward prior to being discharged from the aircraft in a forward direction. The blocker sustains very large loads, is heavy, and the associated mechanisms incur stowage, maintenance, internal performance and reliability penalties. Thrust reversing is typically accomplished by deploying large clamshell-like or segmented annular doors to block the fan flow turning it outward and forward, possibly assisted by guide vanes. These load-bearing surfaces are heavy, incur a high parts count due to variable positioning and stowage, and cause performance penalties during stowed operation due to total pressure and leakage losses. Although the thrust reverser system is used for only a fraction of the airplane operating time, its impact on nacelle design, weight, airplane cruise performance, engine maintenance and aircraft operating expense is significant.
An engine company study indicates that a thrust reverser system accounts for more than 30 percent of total nacelle weight (not including the engine) for an engine with a fan diameter greater than 100 in. This can be as much as 1,500 lb for a GE90 class engine. Estimated thrust reverser weight is on the order of 55 percent of total nacelle weight for a Folker F100 or Gulfstream GV type installation. This accounts for approximately 600 to 800 lbs. of the nacelle weight. In addition, due to leakage and pressure drops attributed to thrust reverser mechanisms, thrust reverser systems have been estimated to increase specific fuel consumption (SFC) by 0.5 to 1.0 percent. This significantly increases the cost of doing business for commercial operators.
High by-pass ratio engines installed on aircraft such as the 737, 747, 767, 777, DC10, MD11 and A300 use fan stream cascade vane thrust reversers which reverse the fan stream to provide reverse thrust. Generally, core flow is not reversed, due to the complexities and unreliability associated with reversing the core flow. The reversed fan stream provides sufficient reverse thrust in most cases.
The fan stream thrust reverser consists of a series of cascade vanes located around the periphery of the fan cowling aft of the fan exit guide vanes. A series of blocker doors isolates the cascade vanes from the fan stream in forward thrust, while a translating cowl covers the external surface of the cascade vanes, providing a smooth exterior surface for low drag. On actuation, a mechanical drive system translates the outer cowling aft, exposing the cascade vanes external surfaces, while at the same time pivoting the internal blocker door aft on links attached to the core cowling, blocking the fan flow and reversing it through the cascade vanes. With the thrust reverser stowed, engine performance penalties result from leakage round the stowed blocker doors, as well as scrubbing drag associated with blocker door actuating links.
A fixed pivot thrust reverser is used with engines that incorporate a compound nozzle exhaust system, that is, a single nozzle for combined core and fan flow. This type of reverser is used on executive and small commercial aircraft, such as the Falcon 10/20/30, Saberliner, Gulfstream GIV Executive and Folker F100/F70 Regionaliner. The fixed pivot thrust reverser consists of two target doors that are integral with the nacelle aft cowl assembly. The stowed reverser forms the external boattail of the nacelle and also the internal shape of the exhaust nozzle. Internal joints of the stowed reverser nozzle employ seals to minimize leakage in a forward thrust mode. For operation in a reverse mode, the reverser doors pivot aft to form a target that blocks and turns the combined core and fan flow to produce the desired reverser reaction force. The target is located far enough behind the jet-pipe exit to minimize suppression (back pressure) of the engine turbine exhaust and fan flows. The doors generally include end plates to assist in turning the reversed flow. Pivoting of the doors between the stowed and deployed positions is by means of a pushrod to each door driven by a single bellcrank idler that keeps the door motions synchronized. The bellcrank is powered by a hydraulic actuator.
Mechanical blockers employed by known thrust reversers cause in-flight engine performance penalties because they are heavy in weight, require stowage which increases nacelle drag, and cause pressure losses to the fan flow due to inefficient seals between the blockers and the main body. In addition, mechanical blockers sustain very large loads and have a high parts count which decreases reliability and increases the need for maintenance. Thus, thrust reversers employing mechanical blockers cause significant increases to operating and maintenance costs of an aircraft.
French Patent 1,030,483, 1953, is directed to a general application of a blowing jet to turn a large stream radially outwardly, into a cascade of vanes. This patent mentions directionality of the jet (slightly forward, normal to stream, slightly aft). The Figure and text describes a device that cannot be integrated into a modern turbofan engine. The jet configuration described requires more compressor bleed flow than is possible. This patent does not mention any other design characteristics, and does not mention anything about vane cascade design.
Canadian Patent 669,492, 1963, applies the blowing jet concept of the French Patent, but adds an aft-translated cowl (transcowl) to restrict downstream nozzle flow. This assists the jet in turning the large stream into a vane cascade, and is a restriction to the generality. This patent does not mention anything about vane cascade design.